[llvm] [ARM] Emit a warning when the hard-float ABI is enabled but can't be used. (PR #111334)

[Oliver Stannard via llvm-commits]

https://github.com/ostannard commented:

I still think the best place to report errors like this is in the frontend or driver, so that the errors can point to exact source files, line numbers, etc. We could probably add some functions to `TargetParser` to help other frontends with these sort of compatibility checks.

However, I'm fine with this as a backup to avoid generating incorrect code when the frontend either misses a check, or is fine with the lower-quality error messages which the backend can create.

https://github.com/llvm/llvm-project/pull/111334